No abstract

Yellow pyroantimonates Pb-Sb, Pb-Sb-Sn and Pb-Sb-Zn were synthesized by solid-state reactions at high temperature and characterized by X-ray diffraction and Raman spectroscopy. The lattice size of cubic pyrochlores increases with Sn and Zn doping and with Pb overstoichiometry, indicating the replacement of Sb 5+ by the larger cations. This fact permits the understanding of the corresponding Raman spectral modifications as a consequence of the changes in the local symmetry of the Sb-O polyhedra, justifying the exploitation of Raman spectroscopy to noninvasively identify structural modifications of pyroantimonate pigments in artworks.

The effectiveness of Raman spectroscopy (using a bench-top system on standard pigments) for the characterisation of modified lead antimonate yellows was demonstrated in the already published Part I. The knowledge gained is employed here for the study of yellow glazes on genuine Renaissance plates with the aim of identifying non-invasively lead pyroantimonate compounds by portable micro-Raman equipment. The investigation was carried out directly on site, at the Victoria and Albert Museum (London, UK) and at the Museo Statale d'Arte Medievale di Arezzo (Arezzo, Italy), combining Raman and X-ray fluorescence analyses. In addition to the spectral features of both unmodified Naples yellow and Sn-and Zn-modified lead antimonatecompounds, the Raman patterns related to partially modified pyrochlore structures were observed. For this reason, the possible Sn-induced modification of Naples yellow by cassiterite (SnO 2 ), present within the glaze as opacifier was explored on lead antimonate yellow glaze mock-ups fired at different temperatures.

Driven by the need to study precious and irreplaceable artworks without compromising their integrity, researchers have undertaken numerous efforts to develop noninvasive analytical tools and methodologies that can provide a chemical description of cultural heritage materials without any contact with the object. The challenge is that artworks are made of complex mixtures, often with heterogeneous and unknown layered materials. Their components must be identified over a range of size scales, from the molecular identification of constituent compounds to the mapping of alteration phases. In this Account, we review recent research in spectroscopic techniques accessible from the mobile laboratory (MOLAB). The lab is equipped with an array of state-of-the-art, portable, and noninvasive instruments specifically tailored to tackle the different issues confronted by archaeologists, curators, and conservators. The MOLAB approach is suitable for studying a variety of objects, from ceramics to manuscripts or from historical wall paintings to contemporary canvases. We begin by discussing issues related to the acquisition and interpretation of reflectance or backscattered spectra from the surface of heterogeneous materials. Then we show how the selectivity needed for the noninvasive identification of pigments in paintings, even in mixtures or in layered matrices, can be acquired by combining elemental information from X-ray fluorescence with molecular and structural insights from electronic and vibrational spectroscopies. Discriminating between original pigments and restoration retouches is possible, even when both comprise similar chromophores, as highlighted in the study of paintings by Jordaens and Raphael. The noninvasive approach permits the examination of a very large number of artworks with a virtually limitless number of measurements. Thus, unexpected and uncommon features may be uncovered, as in the case of a lead pyroantimonate yellow doped with zinc that was discovered by micro-Raman and X-ray fluorescence on an Italian Renaissance majolica. For characterizing binding media, we discuss the strengths and limitations of using mid- and near-FTIR (Fourier transform infrared) spectroscopies supported by a multivariate statistical analysis, detailing the study of organic materials in a wall painting by Perugino and a survey of the painting technique on 18 contemporary paintings by Burri. In Michelangelo's David, we show how the noninvasive mapping of contaminants and alteration phases might inform decisions on preventive conservation plans. The multitechnique MOLAB approach overcomes the intrinsic limitation of individual spectroscopic methods. Moreover, the ability to analyze artworks without the need to move them is an invaluable asset in the study and preservation of cultural heritage.